Switching units, in particular circuit breakers, are used, inter alia, for safe disconnection in the event of a short circuit and thus protect consumers and installations. In addition, electrical or mechanical switching units are suitable for the operationally dependent, manual switching of consumers and for the safe isolation of an installation from the electrical grid in the event of maintenance work or changes to the installation. Electrical switching units are often operated electromagnetically.
That is to say that such switching units or circuit breakers are electrical switching devices which are high quality in technical terms with integrated protection for motors, lines, transformers and generators. They are used at service facilities with a low switching frequency. Such switching units are also suitable for overload protection, in addition to short-circuit protection.
In the event of a short circuit, an electrical switching unit or a circuit breaker disconnects an electrical installation safely. Thus, this electrical switching unit provides safety protection from overload. Any conductor through which the current is flowing is heated to a greater or lesser extent. The heating is in this case dependent on the ratio of the current intensity to the conductor cross section, the so-called current density.
The current density should not become too great since, otherwise, the conductor insulation can be scorched by excessive heating and possibly a fire can be triggered. In order to protect electrical installations from these damaging effects, switching units or circuit breakers are used as overcurrent protection devices.
Circuit breakers have two tripping mechanisms which act independently of one another for the overload and short-circuit protection. Both releases are connected in series. An electromagnetic release which acts virtually without any delay in time performs the function of protection in the event of a short circuit. In the event of a short circuit, the electromagnetic release unlatches a switching mechanism of the circuit breaker without any delay. A switching armature isolates the switching piece before the short-circuit current can reach its maximum value.
Known switching units have a contact slide unit comprising a contact slide and a movable switching piece. The movable switching piece also has electrical contacts. In addition, such switching units have fixed contacts to an electrical line. In a switched-on state, the electrical contacts of the movable switching piece make contact with the fixed contacts of the switching unit or the circuit breaker.
In the event of a short circuit, the electrical contacts of the movable switching piece are removed from the fixed contacts, with the result that the current flow is interrupted. In this process, the movable switching piece is detached from the fixed contacts. Owing to short-circuit disconnections in a switching unit or a circuit breaker, however, after detachment of the movable switching piece, the movable switching piece can be caused to rotate about its longitudinal axis.
If the movable switching piece rotates about its longitudinal axis, this is also referred to as “bridge rotator”. That is to say that the movable switching piece then, after rotation, no longer returns to its initial position, but remains in the rotated position.
Known contact slides of switching units often have two guide systems. These include an internal guide system and an external guide system. The external guide system is used when the switching operation, i.e. the switch-on or switch-off operation, takes place via a switching mechanism of the switching unit or the circuit breaker. In this case, no bridge rotator occurs.
The internal guide system is used in the event of a short circuit when the switching operation is performed via a switching armature, often a plunger, of the switching unit or the circuit breaker. That is to say that, in the event of disconnection owing to a short circuit, the movable switching piece along the internal guide system leads the contact slide, rebounds at the stop faces in the so-called lower part of the switching unit or the circuit breaker and flies back along the internal guide system again. In this case, it flies in the opposite direction to the switching armature or the plunger of the switching unit or the circuit breaker. In this case, it may occur that the movable switching piece and the plunger meet one another outside their center lines, with the result that this can lead to a rotation of the movable switching piece about its longitudinal axis.
If the movable switching piece remains in the rotated state, when the switching unit or the circuit breaker is next switched on the contacts, in particular silver contacts of the movable switching piece and the fixed contacts of the switching unit or the circuit breaker, no longer meet one another, with the result that failure phenomena occur. That is to say that a switching piece which remains in a rotated position is disadvantageous since the circuit breaker is then no longer usable. A non-functioning switching piece and a non-functioning switching unit are disadvantageous for the electrical consumers and the installation in which the switching unit or the circuit breaker is installed.